This invention relates to a method for fabricating a semiconductor device and, more particularly to a method which is capable of improving uniformity of distribution of impurity concentration in a polycrystalline conductor as a diffusion source, a resistor or a conductor.
Generally, it is necessary to form shallow junctions by way of impurity diffusion in order to obtain a semiconductor device with high performance. For example, one of the conventional methods for forming shallow junctions is as follows. A polycrystalline semiconductor thin film is deposited on a semiconductor substrate, and then, the impurity is ion-implanted into the thin film, and thereafter, heat treatment is carried out to diffuse the impurity into the semiconductor substrate, so that the desired shallow impurity diffusion area is formed. Another method by which a resistor with high performance can be obtained is as follows. A polycrystalline semiconductor thin film is deposited on an dielectric substrate, and then, the impurity is ion-implanted into the thin film, and thereafter, heat treatment is carried out and patterning is then carried out through photolithography and etching stages, so that the desired resistor can be formed.
Generally, the size of crystal grains forming polycrystalline conductive film readily varies in the surface of the semiconductor substrate since thermal distribution at the reaction portion becomes non-uniform or infrared rays are reflected on the surface of the semiconductor substrate when the polycrystalline conductive film is deposited. Therefore, once impurity atoms are implanted in such a polycrystalline conductive film, a part of the atoms are supplied to the border of crystal grains, i.e., crystal grain field because of the heat treatment carried out after the implantation, and then are deactivated. In an area of small crystal grains the size of crystal grain field is large so that a lot of impurity atooms are deactivated, and in an area of large crystal grains, the size of crystal grain field is small so that most of the impurity atoms are uniformly distributed into crystal grains and are activated. Therefore, if the size of the crystal grain of polycrystalline conductive film is not uniform, the impurity concentration distribution in the polycrystalline conductive film becomes non-uniform, and thereby, the concentration distrubution of impurity which is diffused into the semiconductor substrate from the film becomes non-uniform. That is, a junction with bad homogeneity is formed. In the case where the film is used as a resistor, the resistance of the resistor becomes non-uniform. This phenomenon decreases yield in manufacturing and the quality of the products.
A pre-annealing method in which the heat treatment is applied to the polycrystalline conductive film before atoms which determine the conductively type are ion-implanted so that the size of crystal grain is made larger and unified, is disclosed in "Journal of Electrochemical Society", Vol. 131, No. 1, pp. 216-217, but has several drawbacks as follows. In the method, the heat treatment of high temperature over 1,000.degree. C. is necessitated to obtain sufficient uniformity. Thereby, there may occur re-distribution of impurities in another impurity diffusion layer which was already formed, and there may occur defects due to stress so that it is not preferable to apply the method to actual processes for fabricating semiconductor devices.
Further, "Journal of Vacuum Science Technology B ", Vol. 2, No. 4, pp. 698-706 discloses that in the heat treated polycrystalline conductive film, the closer to a surface opposite the surface which contacts the substrate, the larger the size of the crystal grain becomes are therefore, uniformity is obtained. If such an idealized crystal phase exists, after amorphizing a surface opposite to a surface which contains the substrate by a conventional way of impurity ion implantation, fine crystal grains of the polycrystalline conductive film located near a surface which contacts the substate, may function as seeds and recrystallize the polycrystalline conductive film because of the heat treatment, so that the size of crystal grain becomes non-uniform and, it becomes difficult to obtain high uniformity of impurity concentration distribution.